Download intrinsic brightness

Module P7 L6
Which is Brightest?
A glow worm 1 m away . . .
. . . or car headlights 1000 m away?
They could both appear to be the
same brightness.
The intrinsic brightness of the glow
worm is 1/1000th of the intrinsic
brightness of the headlights . . .
. . . but the glow worm is 1000 times
closer . . .
. . . so the observed brightness is the
same.
Analysing the Results
NOTE: The resistance of the LDR gets higher in the dark and lower in
bright light
8. What can you say about the intrinsic
brightness of the bulb during this
experiment?
9. What is the relationship between distance
and the resistance of the LDR?
10. What is the relationship between
distance and observed brightness?
Two stars A and B could have the same
observed brightness because:
(1) A and B have the same
intrinsic brightness and
are the same distance
away from us
(3) B has a greater intrinsic
brightness than A but is
further away
(5) A has a greater intrinsic
brightness than B and is
the same distance or
closer but its light is
dimmed by passing
through dust clouds
(2) A has a greater intrinsic
brightness than B but is
further away
(4) A has a greater intrinsic
brightness than B and is
the same distance or
closer but its light is
dimmed by passing
through dust clouds
Colour of a Star
Stars can appear to be blue, yellow or red . . .
Blue stars are the largest and hottest.
Yellow stars are relatively small and cool (our Sun is classified as a
‘yellow dwarf star’).
Red stars could be either:
• red giants (large but cool)
• red dwarfs (small and cool)
Intensity of
radiation at
each frequency
Star light, star bright
The area under the graph represents
the total energy emitted by the star.
Blue stars give off more energy than
red stars.
Smaller Frequency
(Longer Wavelength)
Stars emit light at all frequencies.
However, some stars emit more (say) blue light and so they appear blue.
Real exam
question
[3]
86 light years
from Earth
62 light years
from Earth
Nearby stars also show an
annual motion due to the
movement of the Earth
around the Sun.
The effect is only measurable
when nearby stars are
viewed against a background
of more distant stars.
Parallax
• There is no star (other than the Sun) which has
an annual parallax of more than one second.
• The star with the largest parallax is Proxima
Centauri: 0.77 seconds of arc. (Note: 1 second
of arc = 1/60th of 1 minute of arc = 1/3600th of 1
degree of arc)
• Proxima Centauri is 1/0.77 = 1.295 parsecs
away
• 1 parsec (pc) is 3.26 light years
Cepheid Variables
Some stars are variable stars – their brightness changes over time.
Some stars are Cepheid Variables – their brightness changes in a regular pattern.
The period of the pattern is fixed. The period of Eta Aquilae is 7.2 days.
From the period or frequency, we can calculate their intrinsic brightness.
If we know their intrinsic brightness, we can work out how far away they are.
Cepheid Variables 2
In the 1920s, Edwin
Hubble used Cepheid
variables to calculate
the distances to a
number of galaxies.
He used the red shift to
calculate the speed at
which they were
moving.
Hubble’s Discovery
He found that . . .
. . . the further a galaxy the faster
it moved.
He plotted a graph of:
• speed in kilometres per second
( km / s) on the y-axis against
• distance in megaparsecs (Mpc) on
the x-axis
The graph showed that the velocity of a galaxy is directly proportional to its
distance from us.
Hubble measured the gradient as 120 kilometres per second per Megaparsec.
120 km / s / Mpc
The Hubble Constant
The gradient of this graph was the very first indication that we live inside an
expanding Universe.
Astronomers have named this gradient the Hubble Constant in his honour.
Hubble Constant 
speed of recession
distance
speed of recession = Hubble constant x distance
Modern measurements mean that the Hubble Constant is about
70 km / s / Mpc.
Finding a more exact value for the Hubble Constant will allow us to find out
whether we live in an open or closed Universe – we may be able to predict not
just the future of the Universe, but the future of Time itself . . .
. . . and then, just possibly (in the words of Professor Stephen Hawking) “we
should know the mind of God.”
“If we find the answer to
that, it would be the
ultimate triumph of human
reason - for then we should
know the mind of God.”
Stephen Hawking, Lucasian Professor of
Mathematics, Cambridge University,
writing in A Brief History of Time (p.193)